Hydraulic fracturing techniques are widely used to enhance oil and gas production from subterranean formations. During hydraulic fracturing, a fluid is injected into a wellbore under high pressure. Once the reservoir partition pressure is exceeded, the fracturing fluid initiates a fracture in the formation that grows during pumping. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture, thereby affecting the fracture length and width. The viscosity of most fracturing fluids is generated from water-soluble polysaccharides, such as galactomannans or cellulose derivatives. Linear gels that can be operated at ambient temperature do not have the necessary viscosity for proper proppant transferring at elevated temperature. The use of crosslinking agents or crosslinkers, such as borate, titanate, or zirconium ions, can further increase the viscosity. The gelled fluid can be accompanied by a proppant that results in its placement within the fracture that has been produced. The proppant remains in the fracture to prevent its closure and to form a conductive channel extending from the wellbore into the formation once the fracturing fluid is recovered.
Guar-based fracturing fluids are the most commonly used fluids in reservoir stimulation. As indicated, stimulation of oil and gas wells has been improved by the ability to crosslink the fracturing fluids to increase their viscosity. Some common crosslinking agents include boron and zirconium or other metallic compounds. Boron crosslinked gels are more commonly used due to their reversibility to mechanical shearing and less harmful environmental properties.
Common boron crosslinkers include boron anhydrides, borate esters, and inorganic borates. Generally for boron crosslinkers, the effective crosslinking species is B—OH, often formed after hydrolysis. For crosslinking, the B—OH moieties must be able to come into proximity with the functional groups on the polymer chains to form a chemical bond and bind the chains together.
U.S. Patent Publication Nos. 2010/0099913 A1 and 2010/0099586 A1, herein incorporated by reference, disclosed a family of poly(aminoborates) that can be used as effective crosslinkers. These applications provided a method to synthesize the poly(aminoborates) through a reaction between polyamines and trialkylborates.
However, this reaction is limited by the boiling point of the trialkylborate, usually trimethylborate. The reaction must be performed in a pressure vessel to increase the temperature and reaction rate to allow the reaction to be driven towards completion but prevent evaporation of the trialkylborate. A closed reaction system makes the reaction more difficult to perform and to monitor, which is a concern for quality control. Furthermore, the commonly used, trimethylborate, is moisture-sensitive and highly flammable with a low flash point. The storage of it requires special conditions, especially for humid climates, which adds to the cost of the final product.
It would be a significant improvement to find a reaction route to synthesize polyboronic crosslinker compounds without the shortcomings associated with using trialkylborate reagents.